Many systems use cavity filters to define resonant frequencies for microwave or radio frequency (RF) signals. Such cavities may have an enclosed space surrounded by at least one electrically conductive wall. The dimensions of this enclosed space and the interaction of the electromagnetic waves that embody the signals with the at least one electrically conductive wall define particular frequencies.
A cavity filter is not useful without means for coupling energy into the cavity and from the cavity, so a coupler may be added to transfer a portion of the energy from the cavity filter to an external location. A simple coupler could be a direct metal to metal connection, such that the coupler directly taps energy from the conductive walls of the cavity.
However, such DC-grounded tapping has a number of drawbacks. For example, due to non-linearity in the electromagnetic waves at the metal-to-metal contacts, Passive Inter-Modulation (PIM) signals may appear when signals pass from the cavity walls into a conductive junction. Such degradation in performance is particularly likely when a conductive wall of a cavity is directly linked to a metallic coupler. PIM signals raise a number of issues, including distortion of a desired signal that may potentially degrade system performance.
PIM may be avoided, to some extent, by high quality workmanship, such that the metallic conductor is precisely soldered to a cavity wall. However, even one skilled in metallurgy may be unable to perfectly shape the junction, so some PIM signals will persist. Thus, an alternative solution may be needed that does not involve a metal-to-metal junction.
One alternative is to place a dielectric between the metallic wall of the cavity and the external conductor. Fixed capacitive tapping may use a coaxial structure. However, such a structure is not easily tunable, so it can only tap a set amount of energy from a cavity filter.
Another conventional method requires insertion of tuning screws into a microwave cavity. While rotating a screw to vary the depth of its penetration into the cavity does achieve tuning, it may be difficult to duplicate such tuning when the environment requires adjustment of a very large coupling range with a single design. Thus, it would be beneficial to have a tuning technique for a cavity that was repeatable, resulting in identical coupling each time the technique was used in the same way in a cavity having the same dimensions.
For the foregoing reasons and for further reasons that will be apparent to those of skill in the art upon reading and understanding this specification, there is a need for a capacitive coupling technique that is both easily tunable and adequately reduces PIM.